Sportal.co.nz 22.Nov.2012Getty Images
The All Blacks forward received just a one-week ban for stamping on Scotland flanker Alasdair Strokosch's head during a Test match at Murrayfield between the countries 10 days ago.
Thomson was yellow-carded for the incident, but then subsequently cited, which meant him appearing before independent judicial officer Jean Noel Couraud.
Couraud upheld the citing complaint in a hearing run under the auspices of the Six Nations committee.
The suspension would have been two weeks, but Couraud reduced it by seven days for Thomson's good conduct at the hearing.
It meant he missed last Saturday's clash against Italy in Rome, but made him available to face Wales this weekend, and then England at Twickenham seven days later.
The length of ban sparked criticism from throughout rugby, and in a strongly-worded statement the IRB said tonight it would appeal the Thomson decision 'in the best interests of the Game'.
"As custodians of rugby worldwide, the IRB has a duty to protect its image, values and integrity, together with the welfare of players at all levels in order that the sport can continue its unprecedented growth and welcome more men, women and children to the rugby family," the statement read.
"At the very heart of this mission is the universal application of the disciplinary process as set out in Regulation 17.
"This IRB disciplinary regulation is intended to protect all players and the Game through the strict application of a sanctioning regime that acts as a strong deterrent against acts of foul play.
"This stance was reaffirmed by leading international players, coaches, referees and administrators attending the IRB morality conference in London in March this year, who unanimously agreed that a tough and consistent stance on discipline is key to rugby's integrity.
"After careful consideration and having reviewed the full written decision in the Thomson case well within the permitted 72 hours of receipt, the IRB strongly believes that the sanction of one week is unduly lenient for this particular act of foul play and not aligned with the sanctions handed down in similar cases.
"The IRB firmly believes it is in the best interests of the Game and its integrity to exercise its ability to appeal the Thomson decision.
"The right of appeal by the IRB [in defined circumstances] and for host unions and tournament organisers was introduced into the revised Regulation 17, effective on June 1 this year, to uphold the integrity of the disciplinary process in appropriate cases."
The IRB said arrangements for an appeal hearing will be announced shortly.
Thomson, 30, is available to the All Blacks selectors, who will announce their team on Thursday (UK time) to face Wales at the Millennium Stadium this weekend.